In U.S. Pat. No. 3,726,598 which issued Apr. 10, 1973 to Anthony C. Gilby and James Alan Horton, there is disclosed a multiple pass optical absorption cell. The cell includes a pair of identical spherical objective mirrors at one end and a field mirror at the other. The two objective mirrors are positioned on a common mount such that their centers of curvature are separated by a fixed predetermined distance. The optical path length through the cell is changed by rotating the common mount but the distance between the centers of curvature remains fixed.
The multiple pass cell disclosed in the above-mentioned patent represented an important advance over cells known to the prior art and has been an outstanding commercial success. However, it would be desirable to simplify the construction, particularly, the construction of the objective mirror arrangement. In the patent, the two identical spherical objective mirrors are independently mounted upon an angled mirror mount which maintains the centers of curvature of the mirrors separated by the desired distance. It would be desirable to eliminate this mounting step. The mounting step also introduces the possibility of vertical misalignment and, accordingly, additional rigidity between the two objective mirrors is desirable.
Accordingly, it is a primary object of the present invention to provide a plurality of individual spherical mirrors having their centers of curvature separated in space while avoiding the need for making individual mirror adjustments.
Another object is to simplify the grinding and polishing of such a mirror pair.
Another object is to provide such a pair of mirrors wherein the tendency of the mirrors to tilt vertically is tightly constrained.
Other objects, features and advantages will be apparent from the following description and appended claims.